Linear motors and actuators are now cost-competitive with ball screws and belt drives and offer distinctly superior agility and bandwidth for advanced positioning applications. New micromotors and actuators are helping to automate tasks not previously feasible. Direct linear drives are increasingly replacing servo-controlled pneumatic cylinders, contributing reliability and controllability, free from the cost, noise, and upkeep of air compressors.

Figure 1. Linear motor positioning stages provide a rugged, cost-competitive drop-in upgrade for ballscrew and servo-pneumatic positioning mechanisms.

Driven by semiconductor industry requirements, linear motor manufacturers have steadily increased precision, reduced prices, developed multiple motor types, and simplified integration into automation equipment. Modern linear motors provide 20g peak acceleration and 10-meters/second velocity, deliver unmatched dynamic agility, minimize maintenance, and multiply uptime. They have moved beyond specialized semiconductor industry usage to provide advanced performance in hosts of applications.

With ten times the speed and operating life of ball screws, linear direct drive technology is often the only solution for productivity-enhancing automation.

Dynamic superiority

The dynamic performance of conventional positioning mechanisms is limited by lead screws, gear trains, belt drives, and flexible couplings, which produce hysteresis, backlash and wear. Similarly, pneumatic actuators suffer from piston mass and piston-cylinder friction, as well as air compressibility, which produces servo control complexity. Linear motors and actuators shed the mass and inertia of conventional positioners, and freed from these fundamental limitations, provide unequalled dynamic stiffness.

Direct drive force creation enables linear motors and actuators to achieve closed-loop bandwidth unavailable with alternative positioning mechanisms. The motor and actuator are able to take full advantage of modern controllers. These controllers are tuned for high loop gain operation, achieving wide bandwidth control, fast settling, and rapid recovery from transient disturbances.

Linear motors and actuators excel in making millimeters distance moves that operate in the static friction zone. Their low mass and minimal static friction minimizes the drive force necessary to start travel, and simplify the control system’s task in preventing overshoot when stopping. These attributes enable direct drive motors and actuators to scan microscope slides, for instance, and chart the X-Y locations of artifacts only millimeters apart.

Applications requiring rapid repetitive motion can exploit the linear actuator’s high bandwidth to double the throughput of ballscrews or belt drives. Machines that slice rolls of material to length (paper, plastics, even diapers) maximize throughput by operating without stopping the material flow. To cut on the fly, such machines accelerate the cutting blade to synchronize with material flow, travel at material speed to the cutting location, and then initiate the cut. After cutting, the blade is returned to to its starting point to await the next round-trip cutting cycle.

Linear motor types

Three basic linear motor configurations are available: flat bed, U-channel, and tubular motors. Each motor has intrinsic benefits and limitations.

Flat bed motors, while offering unlimited travel and highest drive force, exert considerable and undesirable magnetic attraction between the load carrying forcer and the motor’s permanent magnet track. This attraction force requires bearings that support the extra load.

The U-channel motor, with its iron-less core, has low inertia, hence maximum agility. However, the forcer’s load carrying magnetic coils travel deep within the U-channel frame, restricting heat removal.

Tubular linear motors are rugged, thermally efficient, and the simplest to install. They provide drop-in replacements for ballscrew and pneumatic positioners. The tubular motor’s permanent magnets are encased in a stainless steel tube (thrust rod), which is supported at both ends. Without additional thrust rod support, load travel is limited to 2 to 3 meters, depending on thrust rod diameter.

Figure 2. Linear servo motors come in flat bed (TOP), U-channel (center), and tubular (bottom) styles.

Of all three motor types, tubular motors are best equipped for mainstream industrial usage. Tubular linear motors have gained profound benefits from a fundamental engineering innovation. Copley Controls’ linear motors replace the traditional external linear encoder with integral Hall sensors. A patented magnetic circuit enables Hall-effect sensors to achieve almost tenfold improvement in resolution and repeatability.

As linear encoders can cost almost as much as the linear motor itself, eliminating them is a major cost reduction. This also simplifies linear motor integration into automation systems, as there’s no finicky encoder to support and align. Other benefits include ruggedness, dependability, and freedom from an encoder’s need for protected environments.

Tubular linear motors can be transformed into powerful, versatile direct drive linear actuators. In an actuator incarnation, the forcer remains stationary (bolted to machine frame), while the load positioning thrust rod travels on low friction, lubrication-free bearings mounted within the forcer. Besides outperforming ball screws and belt drives, the linear actuator is a higher-performance alternative to programmable servo-pneumatic positioning systems.

Tubular linear motors lend themselves to productivity doubling applications with two independent forcers operating on a single thrust rod. Each forcer has its own servo drive, and can travel fully independent of the other. One forcer can then load, for example, while the other unloads. The technique can double throughput by lifting items two at a time from a fast traveling conveyor and place them with precision on a second conveyor.

Similarly, multiple forcers operating on a single thrust rod can double, triple, or even quadruple drive force. The forcers can be operated by a single controller.

Figure 3. Tubular linear motors replace external position encoders with integral Hall sensors, bringing cost, performance and application benefits.

The linear motor load-carrying forcer travels on long-life, single rail bearings. In contrast, ballscrew rotary-to-linear conversion mechanisms involve additional sources of wear that degrade performance and shorten life.

The linear actuator thrust rod glides on long-life, lubrication-free bearings mounted in the forcer. This intrinsic simplicity enables the actuator to deliver 10 million operating cycles. Actuator bearings are self-aligning, easing installation. The actuator drive force is applied directly to the thrust rod, improving acceleration and responsiveness.

With the external encoder replaced by a solid state sensor integrated into the forcer, direct drive motors and actuators become very simple two-component devices. Forcer and thrust rod are both inherently very robust components, which enables motor and actuator to conform to international IP67 washdown ratings.

The absence of grinding gears and whirring lead screw gives linear motors and actuators the increasingly vital qualification of low noise operation. OSHA is following close on the heels of European industrial codes, which place increasingly stringent rules on workplace noise. Quiet operation is already critical in laboratory and hospital environments; this concern will become increasingly widespread as OSHA extends its ruling to other production environments.

This article was written by George Procter, vice president of Copley Motion Systems LLC, Canton, MA. For more information, Click Here 


Motion Control Technology Magazine

This article first appeared in the April, 2008 issue of Motion Control Technology Magazine.

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